The purpose of this study is to investigate why some people develop life-threatening infections caused by the bacteria Staphylococcus aureus, while other people do not. It is possible that the infectious ability of the bacteria can determine whether an infection develops and its severity. The investigators will look at old blood and nasal specimens collected from 1000 adults who had S. aureus infections and who were hospitalized at Duke University Medical Center. Previously collected health information regarding these patients and the specific bacterial traits in the samples will be studied. Eventually this information may be used to help treat and prevent S. aureus infection.